1. Field of the Invention
This invention relates to an apparatus for encoding an input digital signal.
2. Description of the Prior Art
For high efficiency encoding of audio or voice signals, a known encoding technique employs adaptive bit allocation in which the input signals, such as audio or voice signals, are divided on the time axis or on the frequency axis into a plurality of channels and the numbers of the bits are adaptively allocated for the respective channels. Among the techniques for encoding audio signals by adaptive bit allocation are a sub-band coding (SBC) technique, in which the audio signals on the time axis are divided into a plurality of frequency bands for encoding, an adaptive transform coding (ATC) technique in which signals on the time axis are converted into signals on the frequency axis by orthogonal transform and divided into a plurality of frequency bands in each of which adaptive signal encoding is performed, and a so-called adaptive bit coding (APC-AB) technique, in which the sub-band encoding and adaptive transform encoding techniques are combined such that the signals on the time axis are divided into frequency bands and the signals of the respective bands are converted into baseband signals and predictively encoded by nth order linear predictive analyses, where n is an integer equal to two or more.
In the field of high efficiency encoding, techniques in which the so-called masking characteristics of the human auditory sense are taken into account, have been adopted extensively. The masking effect refers to a phenomenon in which a signal is masked by another signal and thereby becomes inaudible. This masking effect is classified into the masking of audio signals along the time axis and the masking of signals on the frequency axis.
The masking of audio signals on the frequency axis is now explained. In the case of a sine wave W.sub.S having a frequency f.sub.s, the masking spectrum or masking curve MS, representing the masking effect by the human auditory sense, is as shown in FIG. 1. The illustrated masking spectrum MS signifies that the area indicated by hatched lines is masked. Thus the noise within the masking spectrum MS, if any, becomes inaudible, so that, in the actual audio signal, any noise within the masking spectrum MS is allowable. Thus the allowable noise level in the case of the sine wave W.sub.S is below the level shown at j in FIG. 1. Also the masking effect is maximum at the frequency f.sub.s of the sine wave W.sub.S and becomes lower the more the frequency differs from the frequency f.sub.s of the sine wave W.sub.S.
The masking of audio signals along the time axis is classified into temporal masking and concurrent masking. Concurrent masking means an effect in which a smaller sound (or noise) generated simultaneously with a larger sound is masked by the larger sound and becomes inaudible. Temporal masking means an effect in which, as shown in FIG. 2, the smaller sound or noise temporally before and after a larger sound (a high level signal part C in the figure) is masked by the larger sound and becomes inaudible. With temporal masking, masking of the sound temporally after the larger sound is termed forward masking and masking of the sound temporally before the larger sound is termed backward masking. In temporal masking, the effect of forward masking FM in FIG. 2 persists for a long time (of the order of 100 msec, for example), while that of backward masking BM has a shorter duration (of the order of 5 msec, for example), on account of the characteristics of the human auditory sense. The masking level or masking quantity is of the order of 20 dB and 30 dB with forward masking and with backward masking, respectively.
Meanwhile, with the above described high efficiency encoding techniques, it is desired to achieve a further increase in the bit compression rate L or the amount of bit reduction. However, in such techniques bit compression is achieved by taking advantage only of either the masking effect on the frequency axis or the masking effect along the time axis. That is, both masking effects have not been utilized simultaneously.